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AWD 1 Tire replacement needed != 4

M

Mide

Golden Member
So I've heard a lot of things in terms of tire replacement. If you have a FWD car and 1 tire goes out, you'll need to replace 2. If you have an 4x4 or AWD car and 1 tire goes out you'll have to replace 4.

It seems like bullshit to me because on an AWD car, usually your 2 front tires are already more used than your 2 rear ones. The same goes for if you make a ton of right turns on a daily basis. Rotating your tires every 2 oil changes helps with the leveling out of the tires' ware, but still.

So what would actually, realistically happen to your car if you only replaced a single tire on a FWD or AWD car? All the car places I've gone too would not do the service of only 1 tire because it was deemed a safety risk. I think that is BS...they are out to get more money. Yes, it may be better for leveling purposes (replacing 2 front ones on a FWD), but I don't think it will cause irreversible damage. Of course I'm not a mechanic so that's why I'm asking.
 
You'll likely do major damage to differentials by driving with a tire of significantly different diameter.

You can just buy 1 tire and have it shaved down if that particular tire model is still available.
 
Consult your owner's manual to make sure you are within spec. That is the correct answer.

As far as the real world answer, it doesn't matter. I've never seen a differential damaged due to a replacement tire in an AWD vehicle. The rotational variance in turns alone are greater than the difference in 2/32" of tire diameter.
 
caspur said:
Consult your owner's manual to make sure you are within spec. That is the correct answer.

As far as the real world answer, it doesn't matter. I've never seen a differential damaged due to a replacement tire in an AWD vehicle. The rotational variance in turns alone are greater than the difference in 2/32" of tire diameter.
Click to expand...

Turning will only cause your differentials to work while you're cornering and unless you drive in Nascar I'd assume that the number of right turns you do equals the number of left turns you do.

If you have a tire that is different on a driven wheel the differential will always be working like you're going around a turn rather than it basically being along for the ride when you're going straight. A few thousand miles constantly working your differential seems like it could cause more wear than a well matched set of tires.

Also, if your front wheels are wearing more than the rears its time to start rotating your tires. That will even out the wear and give you longer intervals between visits to the tire shop.
 
4x4 = If you change one tire on front, you need to replace both. You don't need to replace all 4.

Btw 4x4 systems are not all biased to the front. in my parents car 60% of power goes to the rear wheels.
 
4X4/4WD != AWD.

all wheel is a system that can't be disengaged with the front wheels typically being the primary drive. four wheel has a switch and is always based on a RWD setup.

the two tire thing is actually more bullshit than saying an AWD needs four tires. unless the car has a limited slip differential, you could have two totally different tires on each side and the drive train wouldn't care a bit (ABS would). running tires with a different diameter between the front and rear could tear up a viscous coupling. not sure what the effect would be on an electronically managed AWD system, but i would avoid it in that situation, too.

how much difference is too much is really the only point of debate. it would be much better to be safe than sorry- if the car needs tires, get all four, learn to rotate regularly, and you won't have to make this decision again.
 
The reason for the two tire rule isn't because of a diameter difference, its for traction purposes. Even the same tire, with different levels of wear, will have different traction/handling characteristics. Think about it, would you want a street car that turns left better than right? What about going up a mountain road? Bad idea especially when rain/snow comes in to the equation.

Still think its a stupid rule? Merely a ploy to sell you more tires?
 
are you addressing me?

the question was whether it would hurt the car. in the case of the 'two tire rule,' no, it generally will not. i really don't care about your speed racer tire physics, i see far less danger in having four different brands of tire on a car than the jackasses that drive around on matched sets of bald and/or improperly inflated tires.
 
Sukhoi said:
You'll likely do major damage to differentials by driving with a tire of significantly different diameter.

You can just buy 1 tire and have it shaved down if that particular tire model is still available.
Click to expand...

How? A differential's job is to allow the tires to spin at different speeds. In your example, the speed difference is caused by different tire size.
 
Proper operation of four wheel drive vehicles depends on
tires of equal size, type, and circumference on each
wheel. Any difference will adversely affect shifting and
cause damage to the transfer case.
Click to expand...

From my full time all wheel drive 2008 Grand Cherokee owner's manual. It's normally 52% torque to the rear and 48% to the front, but can go 100% to the front or rear.
 
biggestmuff said:
How? A differential's job is to allow the tires to spin at different speeds. In your example, the speed difference is caused by different tire size.
Click to expand...

Yes, but when you're going straight the spider gears don't spin. If you have a wheel that is different size your spider gears will be turning constantly, even when you're going straight. That will drastically increase the number of turns that the gears will see in the same amount of time.
 
NutBucket said:
Still think its a stupid rule? Merely a ploy to sell you more tires?
Click to expand...

Most peoples' feedback seems to make sense. But yes I still think it's a stupid rule. If it's a traction issue then I'd rather deal with the uneven traction instead of paying for the purchase and installation of an additional tire.

If the statements about the differential are correct then fine, that is a valid-enough reason to pay up.
 
brblx said:
4X4/4WD != AWD.
the two tire thing is actually more bullshit than saying an AWD needs four tires. unless the car has a limited slip differential, you could have two totally different tires on each side and the drive train wouldn't care a bit (ABS would).
Click to expand...

Very wrong. An open diff would care VERY much if the spider gears were constantly doing work when they shouldn't.
 
yeah, and you should keep your motor at idle all the time, because making it turn more than it needs to is going to ruin it. 🙄

the spider gears are meant to turn. there is always a load on them. you're not going to wear them out. what you've been told is that two different size tires will hurt an LSD (they will) and you've somehow corrupted that knowledge. go away.
 
Hmmm.... I was needing to know this info myself. I have a couple of tires with slow leaks, one worse than the other (about a week and it is at 0psi). They're both on the driver side. I was wondering about taking the two on the passenger side and putting them on the back and two new ones on the front (yeah, it's fwd).
 
on my civic SI, i'm never supposed to have diff tires on the front L/R due to the LSD.

if i get a flat tire in the front, i have to move the rears up to the front, and place the good front and spare at the rear.
 
thescreensavers said:
explain so I may learn.. I dont see anything wrong in what I said.
Click to expand...

You're confused about AWD and 4x4. There's also the issue that was addressed multiple times already, where mismatched tires will cause the spider gears in the differential to continue turning, even when the car is going straight. This drastically increases the revolutions they would see in the same period of time.

4x4 is not the same as AWD. AWD includes a central differential and is typically engaged all the time. This center differential allows the front wheels to spin at a slightly different rate than the rears which will occur as you go around turns.

A true 4x4 vehicle will have a way to disconnect two of the wheels for normal driving on the road. This can be manual or can be automatic in some vehicles that will engage it when it senses a low traction situation. If you engage a true 4x4 system in situations where you have high traction like on dry pavement you can actually put quite a bit more stress than you should on the transfer case. However, this has an advantage for offroad vehicles. If you completely lift the front or rear wheels off the ground and give it gas the other wheels have to turn. There's no center differential that controls the torque split front to rear, if the engine's turning there's always power getting to the axles. There's not really anything there to control how much power that's going to the front or the rear.
 
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